The rapidly evolving hypergiant IRC+10 420: High-resolution bispectrum speckle-interferometry and dust-shell modelling

The hypergiant IRC +10 420 is a unique object for the study of stellar evol ution since it is the only object that is believed to be witnessed in its r apid transition from the red supergiant stage to the Wolf-Rayet phase. Its effective temperature has increased by 1000-2000 K within only 20 yr. We pr esent the first speckle observations of IRC +10 420 with 73 mas resolution. A diffraction-limited 2.11 mu m image was reconstructed from 6 m telescope speckle data using the bispectrum speckle-interferometry method. The visib ility function shows that the dust shell contributes similar to 40% to the total flux and the unresolved central object similar to 60%. Radiative transfer calculations have been performed to model both the spect ral energy distribution and visibility function. The grain sizes, a, were f ound to be in accordance with a standard distribution function, n(a) simila r to a(-3.5), with a ranging between a(min) = 0.005 mu m and a(max) = 0.45 mu m. The observed dust shell properties cannot be fitted by single-shell m odels but seem to require multiple components. At a certain distance we con sidered an enhancement over the assumed 1/r(x) density distribution. The be st model for both SED and visibility was found for a dust shell with a dust temperature of 1000 K at its inner radius of 69 R*. At a distance of 308 R * the density was enhanced by a factor of 40 and and its density exponent w as changed from x = 2 to x = 1.7. The shell's intensity distribution was fo und to be ring-like. The ring diameter is equal to the inner diameter of th e hot shell (similar to 69 mas). The diameter of the central star is simila r to 1 mas. The assumption of a hotter inner shell of 1200 K gives fits of almost comparable quality but decreases the spatial extension of both shell s' inner boundaries by similar to 30% (with x = 1.5 in the outer shell). Th e two-component model can be interpreted in terms of a termination of an en hanced mass-loss phase roughly 60 to 90 yr (for d = 5 kpc) ago. The bolomet ric flux, F-bol, is 8.17 . 10(-10) Wm(-2) corresponding to a central-star l uminosity of L/L. = 25 462 . (d/kpc)(2).